Industrial material handling vehicles such as fork lift trucks or motorized hand pallet trucks are commonly found in warehouses, factories, shipping yards, and, generally, wherever pallets, packages, or loads of goods are required to be moved from place to place. Pallet trucks typically include a load bearing fork or lift arm for lifting packages or pallets to a height sufficient for transporting, an electric drive motor for driving the vehicles, a steering control mechanism, and a brake. These vehicles can include an operator station, on which the operator stands as the pallet truck moves, or can be designed for the operator to walk behind the vehicle at the end opposite the forks.
The steering mechanism for a common type of pallet truck includes a movable arm or tiller and a control handle mounted at the end of the tiller. The tiller is rotatable right and left to steer the vehicle, while a rotatable thumb wheel or twist grips on the handle control the speed and direction of the truck, selecting between a forward and a reverse direction. To prevent movement of the truck when the operator has left the vehicle, the steering tiller arm is typically spring loaded. When the tiller is released, it is forced by the spring to a near vertical position outside of a defined operating arc. In the vertical position, a spring-applied “deadman” brake mechanism is automatically activated to prevent further motion of the vehicle.
To activate the deadman brake quickly and to limit use of the vehicle when the tiller is in a near vertical position where the mechanical advantage for steering is typically poor and the potential speed of the vehicle is nonetheless relatively high, prior art material handling vehicles were constructed to require the tiller arm to be moved a relatively large angle from the vertical position prior to releasing the brake. The steering arm or tiller therefore had to be pulled a significant distance toward the horizontal before operation of the vehicle was allowed. Limiting operation in this way, however, poses problems for pallet trucks used in narrow lanes and, in an increasingly common mode of operation, inside of trailers and other large containers for moving goods. In these applications, the horizontal space available for swinging the tiller to the right and left within the operating arc is limited, and it can therefore be difficult to steer the vehicle when the tiller is pulled too far from the vertical position. In such applications, it is therefore desirable to allow the vehicle to be controlled with the tiller in a nearly vertical position.
One solution to this problem has been to provide switches in conjunction with the steering tiller which are activated to indicate a transition to a first angle at which a slow mode is entered and a second angle at which a fast mode is entered. In these prior art devices, the pallet truck transitions to a slow mode in which the maximum speed of the vehicle is restricted whenever the first switch is activated, and to a fast mode whenever the second switch is activated, irrespective of the position of the other switch. While typically providing the appropriate functions, these prior art devices suffer from a number of disadvantages. Specifically, when using this type of switching system, it is possible to identify only three driving states, even though two braking states, a fast, and a slow mode are required. Furthermore, as state changes occur irrespective of the position of the other switch, it is difficult to determine when a failure has occurred, or to adequately monitor changes in driving states.